## Categories with Zero Morphisms

Todd Trimble cleared up the confusion with pointed sets and zero morphisms. The problem is that my sources were wrong to say that the monoidal structure on is the categorical product. First, here’s the answer in Todd’s own words:

The zero morphism statement is fine as stated. It just

follows from the fact that the standard monoidal product

on pointed sets is smash product.FWIW, here are details. Recall that the smash product

of two pointed sets A, B (with basepoints 0_A, 0_B) isA /\ B = A x B / ({0_A} x B) \/ (A x {0_B})

where the indicated quotient means the denominator is

identified with a point (and we define the basepoint of

A /\ B to be this point).If C is a category enriched in pointed sets, and a and b

are objects of C, we define the zero morphism 0_ab

from a to be to be to be the basepoint of hom(a, b).

Notice that compositionhom(b, c) /\ hom(a, b) –> hom(a, c)

takes a pair (g, 0_ab) [similarly, a pair (0_bc, f)] to 0_ac,

by definition of smash product. Hence, composition of

a zero morphism with another morphism, on either side,

yields a zero morphism.

Truth be told I’d been thinking about these “smash products”, but they aren’t the regular categorical product I’d been assured worked.

Okay, let’s go over this a bit. Given pointed sets and we define their smash product as follows. First we take everything in *other* than and call it . Similarly, we say is everything in except the point . We take the product and then throw in the new point , which we use as the new special point.

Another way to look at it is to take the regular product , but to “smash” it down a bit. We say that every pair of the form or is “really the same”, and smash all of them together into one special point.

The identity object for this monoidal structure on is the set , as you should check. Also verify that the smash product is associative (up to isomorphism, naturally).

Now a -category has a special morphism in each hom-set, which we’ll (leadingly) call . If we take any other arrow , together they form the pair . But since is the special point of that set, this pair (and any other pair of the form or is the marked point of the smash product. Then the composition function has to send this special point to the special point . Voila: zero morphisms. Compose one with anything and you get another zero morphism back.